Ion exchange can be defined as the reversible interchange of ions between a solid (an ion exchange resin) and a liquid (usually an aqueous solution) in which there is no permanent change in the structure of the solid. Ion exchange resins are synthetic resins containing active substituents (usually sulfonic, carboxylic, phenolic, phosphonous, phosphonic or substituted amino groups) that give the resin the property of combining with or exchanging ions between the resin and a solution. For example, a cation exchange resin with active sulfonic groups in the hydrogen form will exchange its hydrogen ions with, for example, the sodium, calcium, magnesium and other metal ions present in water.
The customary procedure is to pass the liquid through a bed of the ion exchange resin, which is a granular and porous solid and has only a limited capacity for exchanging ions. When the useful exchange capacity of the cation exchange resin is exhausted, the resin can be regenerated with an acid, preferably a strong acid, which removes the accumulated metal ions. Simultaneously, the cation exchange resin takes on an equivalent amount of hydrogen ions in exchange, thus restoring itself to the original hydrogen form. The acid generally used for this regeneration is dilute sulfuric acid. Hydrochloric acid is also an excellent, but usually more expensive, regenerant. An anion exchange resin can be regenerated with a strong base, such as sodium hydroxide.
Mixed bed systems and precoated filter systems containing both anion and cation exchange resins have been used in many industrial applications for the purification of aqueous solutions. A primary application of such systems is in the purification of water for condensate recirculation systems used to drive steam turbines. It is essential that the water be extremely pure to avoid any adverse effects on the surfaces of blades, boilers and pipes of the high pressure steam system. Since it is desired to produce water that is free of any residue upon evaporation, the cation exchange resin should preferably be in the hydrogen form, and the anion exchange resin should be in the hydroxide form.
As used herein, the term "bed" refers to a layer of filtration or ion exchange material, such as a column of ion exchange resin beads or a precoat layer, which has been deposited on a filter support including a filter screen, an annular filter cartridge, a film, a deep or shallow bed and the like. In general, a shallow bed is preferred over a deep bed so that the pressure drop within the bed is minimized.
A particular problem with mixed bed ion exchange systems of the type conventionally employed is the production of ion "leakage," particularly sodium ion leakage. The term "leakage" refers to any ions that are not removed from the water by the ion exchange resin and, thus are permitted to "leak" past the ion exchange resin. As used herein, the term "leakage" also refers to any undesired ions, such as sodium, which are introduced into the water by elution of ions from the ion exchange resin which have not been removed during regeneration.
In a mixed bed system, the leakage problem arises primarily from the difficulty of obtaining perfect separation of the anion and cation exchange resins in the bed before regeneration of the ion exchange resins. As used herein, the term "separation" refers to the bulk classification of ion exchange resins within a single vessel or zone. As is familiar to those skilled in the art, such separation is usually accomplished by passing water upwardly through the ion exchange resins. This stream of water stratifies the mixture of the ion exchange resins by carrying the less dense strongly basic anion exchange resin to the top of the separation vessel, while the more dense weakly acidic cation exchange resin remains in the bottom portion of the vessel.
While the above method is effective for separating the bulk of the ion exchange resins, perfect separation cannot be achieved. A primary source of this difficulty is the ion exchange resin fines produced during handling of the ion exchange resins. Since upflow separation depends upon particle size along with density, the cation exchange resin fines will not sink to the bottom of the separation vessel, but will be carried upwardly with the strongly basic anion exchange resin. When the two ion exchange resins are subsequently isolated from one another, and the anion exchange resin is regenerated with sodium hydroxide, sodium ions will be introduced into the ion exchange sites in the cation exchange resin contaminant. When the ion exchange resins are returned to the service column, the sodium ions will be introduced into the water being treated, producing sodium leakage.
The leakage problem is aggravated when substances such as ammonium hydroxide, ammonia or volatile amines are added even in trace amounts to the steam of a high pressure boiler or turbine system as is conventional to minimize corrosion. The ammonium ions gradually convert the cation exchange resin to the ammonium form and promote the release of sodium ions. Moreover, the amines often appear in the condensate and reduce the capacity of the cation exchange resin to remove corrosion products and traces of ions produced by leakage from the condensate.
In addition, the use of a bed including a strongly acidic cation exchange resin requires the use of a large excess of fresh or unused acid to regenerate the resin. That disadvantage can be overcome, according to the present invention, by use of a weakly acidic cation exchange resin in lieu of a strongly acidic cation exchange resin. As will be described in greater detail with reference to the Detailed Description of the Invention, the effective application of a weakly acidic cation exchange resin in a mixed bed system is contrary to the results one skilled in the art would expect upon reviewing the published literature.